The present invention relates to a color gamut compression device and method therefor which is applied to color inside a color gamut of an information-input apparatus. The color gamut compression device compresses the color gamut by converting color outside a color gamut due to an information-output apparatus into color inside a color gamut due to an information-output apparatus.
Information apparatus such as displays, printers and scanners which process color image data have a range of color which may be input or output which is characteristic to that information apparatus. That is to say, such apparatus have a color gamut. A color gamut in information-output apparatus such as displays and printers may contain a color gamut of an information-input apparatus such as a scanner or a display. In such a case, it is possible to express the color of the image from the information-input apparatus without change in the information-output apparatus. However when a section of a color gamut in an information-output apparatus is not contained in the color gamut of an information-input apparatus, a source color inside a color gamut in the information-input apparatus can not be expressed by color outside the color gamut in the information-output apparatus without change by the information-output apparatus. Thus color which is outside the color gamut of the information-output apparatus is output after conversion to a color inside a color gamut of the information-output apparatus. That is to say, when a color gamut of an information-output apparatus is not coextensive with a color gamut of an information-input apparatus, color gamut compression processing is required.
FIG. 1 to FIG. 4 explain a conventional color gamut compression method such as disclosed for example in JP-A-9-98298.
The color gamut compression method shown in FIG. 1 is a saturation method. In a saturation method, with respect to color inside a color gamut of an information-input apparatus, color which is outside the color gamut of the information-output apparatus is converted to color inside a color gamut of the information-output apparatus by converting only saturation and without converting the brightness.
The color gamut compression method shown in FIG. 2 is a brightness compression method. In a brightness compression method, with respect to color inside a color gamut of an information-input apparatus, source color which is outside the color gamut of the information-output apparatus is converted to a target color inside a color gamut of the information-output apparatus by converting only brightness and without converting saturation. When a color with the same saturation as the color in the information-input apparatus is not present in the color gamut of the information-output apparatus, that is to say, when the source color of the information-input apparatus is positioned in the section of the curved line in the figure, the source color of the information-input apparatus is converted to the same fixed color of the information-output apparatus.
The color gamut compression method shown in FIG. 3 is a color difference minimization method. In a color difference minimization method, color outside the color gamut of the information-output apparatus with respect to a color inside a color gamut of an information-input apparatus is converted to a color with the smallest color difference from the color in the information-input device. Thus color in the information-input device on the curved line in the figure is all converted to the same color.
Color gamut compression processing can be performed by use of the above method. However it is possible that if employed in a saturation compression method. The tonal range in the high saturation regions will be improved to an extent, the overall effect will be an image without color definition. Although there is almost no reduction of saturation in a brightness compression method. The color in high brightness regions is converted to low brightness color and the color in low brightness region is converted to high brightness color. Thus brightness after conversion increases with increases in saturation. Furthermore since color conversion is executed in a color difference minimization method in order to minimize color differences, an image which is comparatively close to the image of the information-input apparatus is obtained by the information-output apparatus. However the section on the curved line in FIG. 3 is converted to the same color.
In this way, when using color conversion in the above method, there is a possibility that an unnatural image will be output by the information-output apparatus. A conventional color gamut compression method is proposed for example in JP-A-9-98298 as shown in FIG. 4. In this color gamut compression method, a boundary is formed by a first straight line and a second straight line. The first straight line passes through a first fixed point which has a maximum L_th brightness of a color which has a maximum saturation C_max in a color gamut of an information output device and a second point on a non-saturated axis and which has a minimum brightness L_min in an information output device. The second straight line is formed by a line passing through the first point and a third point on a non-saturated axis and which has a maximum brightness L_max in an information output device. A color gamut outside the information output device with respect to the color gamut of the information input device is divided into low brightness regions 42 and high saturation regions 43. Color in the high brightness region 41 is compressed towards the second point. Color in the low brightness region 42 is compressed towards the third point and color in the high saturation region 43 is compressed towards the first point.
With respect to the color gamut of the information-input apparatus, regions outside the color gamut of the information-output apparatus are divided into a high brightness region 41, a low brightness region 42 and a high saturation region 43. The color of the high brightness region 41 is compressed towards the second point, the color of the low brightness region 42 is compressed towards the third point and the color of the high saturation region 43 is compressed towards the first point.
Apart from the above method, a device for executing color gamut compression processing is proposed in JP-A-7-236069 and JP-A-9-130622 and a device for executing image processing is proposed in JP-A-7-21356.
However as the conventional color gamut compression method is arranged as discussed above it is necessary to determine, with respect to color inside a color gamut of the information-input apparatus whether color outside the color gamut of the information-output apparatus is positioned in any of the high brightness region 41, the low brightness region 42 or the high saturation region 43 and whether to convert such color into color in a compression direction which depends on the region. Therefore the problem has arisen that such a calculation for such a conversion is complicated. Furthermore the problem has arisen that viewpoint of the image after conversion is altered in response to the position of the first point discussed above.
The present invention is proposed to solve the above problems and has the object of providing a color gamut compression device and color gamut compression method which executes the color gamut compression processing by converting source color of an information-input apparatus to a target color inside a color gamut of the information-output apparatus while maintaining the continuity of the color. The conversion is performed by a simple calculation which does not entail a determination of the region in which the color of the information-input apparatus is positioned. Such regions originate in the division of regions outside the color gamut of the information-output apparatus. Color in the information-input apparatus which is not in the color gamut of the information-output apparatus is converted to color inside a color gamut of the information-output apparatus in the direction of a point of convergence. The point of convergence is determined to one of a point at which saturation takes the value of 0, a point of an average value of the color gamut of the information-input apparatus, a point of the central value of the color gamut of the information-input apparatus, and a point of the weighted value of the color gamut of the information-input apparatus.
In the color gamut compression method of the present invention, color in the information-input apparatus which is not in the color gamut of the information-output apparatus is converted to color inside a color gamut of the information-output apparatus in the direction of a point of convergence. The point of convergence is determined to one of a point at which saturation takes the value of 0, a point at an average value of the color gamut of the information-input apparatus, a point at a central value of the color gamut of the information-input apparatus, and a point at a weighted value of the color gamut of the information-input apparatus. The point of convergence is taken as an end point and color is converted to a color of a color gamut in that of the information-output apparatus is positioned on a half line which passes through a point which corresponds to the color of the information-input apparatus. In this way, it is possible to convert color of an information-input apparatus to color inside a color gamut of an information-output apparatus with a simple calculation while maintaining the continuity of the color. Thus color gamut compression which produces a balanced image with respect to both tone and saturation may be performed.
In the color gamut compression method of the present invention, color in the information-input apparatus which is not in the color gamut of the information-output apparatus is converted to color which corresponds to an intersection point of a half line with a contour. The half line passes through a point corresponding to color of the information-input apparatus using a point of convergence as an end point and the contour is of the color gamut of the information-input apparatus in any tone. In this way, it is possible to execute color gamut compression processing which is balanced with respect to both tone and brightness.
In the color gamut compression method of the present invention, it is sometimes the case that a plurality of intersection points exist between a contour of a color gamut of an information-output apparatus and a half line which passes through a point which corresponds to a color of an information-input apparatus using a point of convergence as an endpoint. In this case, with respect to the plurality of intersection points, color of the information-input apparatus which is not in the color gamut of the information-output apparatus is converted to color in which the value of the coordinate r in the polar coordinate system corresponds to the largest intersection point. In this way, it is possible to execute conversion with little variation in the degree of saturation.
In the color gamut compression method of the present invention, a source color of an information-input apparatus which is not in the color gamut of an information-output apparatus is converted to a target color according to three criteria. The first criterion is that a target color must reside within a predetermined angular range of the angle (xcex8, xcfx86) of a source color in a polar coordinate system of a color space. The second criterion is that the target color should be one of a plurality of colors on a contour of a color gamut produced by the information-output apparatus in any tone. The third criterion is that the target color should be obtained by dividing by kH, kV and kC, respectively, the H axis component, the V axis component and the C axis component of a color which minimizes color difference xcex94E given by the equation (1) below, where kH, kV and kC indicate predetermined coefficients corresponding to the H axis, the V axis and the C axis of a HVC color space, and xcex94H, xcex94V and xcex94C indicate an amount of displacement due to conversion of a color component in the H axis, the V axis and the C axis.
xcex94E={square root over ( )}(kHxc3x97xcex94H2+kVxc3x97xcex94V2+kCxc3x97xcex94C2)xe2x80x83xe2x80x83(1)
In this way, it is possible to convert color after conversion to color having characteristics based on the coefficients kH, kV, kC.
In the color gamut compression method according to claim 5, the coefficients kH, kV, kC are set on the basis of Formula (2) when normalized with the maximum values NH, NV, NC of the H, V, C axes of the color gamut of the information-output apparatus.
(kH/NH) greater than (10xc3x97kV/NV) greater than (10xc3x97kC/NC)xe2x80x83xe2x80x83(2)
In this way, it is possible to convert color after conversion to color with a high subjective value based on the coefficients kH, kV, kC.
In the color gamut compression method of the present invention, an intersection point is defined by the intersection of a half line, with an endpoint at a convergence point, passing through a point which corresponds to color of an information-input apparatus with a contour of a color gamut of an information-output apparatus in any tone. With respect to a number of fixed points on a curved surface which is formed by a contour of a color gamut in an information-output apparatus in each tone, the points of intersection are calculated by interpolation based on at least three points from the smallest-value of the difference of the angle (xcex8, xcfx86) with the point corresponding to the color of the information-input apparatus in a polar coordinate system (r, xcex8, xcfx86). In this way, intersection points of a half line passing through a point which corresponds to color of an information-input apparatus and has a point of convergence as an endpoint with a contour of a color gamut of an information-output apparatus in any tone can be more accurately calculated.
In the color gamut compression method of the present invention, when a tone of a color of an information-input apparatus is a tone comprising any of a plurality of tones of a fixed number of standard colors, the tone is taken to be the corresponding tone from the plurality of tones and its brightness is taken to be equal to the brightness of the color with the highest saturation in a color gamut of an information-output apparatus for that tone. Furthermore a point where saturation takes a value of 0 is taken to be a convergence point. When the tone of the color in the information-input apparatus is an intermediate tone with respect to the tones of a fixed number of standard colors the point which is interpolated from the convergence point which corresponds to a tone of a fixed number of standard colors based on tones of colors of an information-input apparatus is taken to be the point of convergence. In this way, for example, the quality of the brightness of a standard color, distributed for example in low brightness, of a color from the blue group is reflected on the color after conversion. Furthermore it is possible to execute color gamut compression processing with higher continuity in the direction of tone.
In the color gamut compression method of the present invention, a vector which corresponds to a color in the information-input apparatus is defined as P and a vector which corresponds to a convergence point is defined as Po. A vector which corresponds to the intersection point of the half line which passes through a point which corresponds to the vector P taking vector P as an endpoint and a curved surface which is formed on a contour of a color gamut in an information-output apparatus in each tone is taken as Ps. The vector which corresponds to the intersection point of the half line and the curved surface which is formed on the contour of the color gamut of an information-input apparatus in each tone is taken to be Pt. When the above conditions exist, the color of a information-input apparatus which corresponds to the vector P is converted to a color which corresponds to the vector Pc by Formula (3) when k has a value which is a fixed constant greater than 0.
Pc=Po+(Pxe2x88x92Po)xc3x97|Psxe2x88x92Po|/|Ptxe2x88x92Po|xc3x97k
(when kxc3x97|Pxe2x88x92Po|xe2x89xa6|Psxe2x88x92Po|)
Pc=Ps
(when kxc3x97|Pxe2x88x92Po| greater than |Psxe2x88x92Po|)xe2x80x83xe2x80x83(3)
In this way, conversion proximate to the contour with respect to colors in the proximity of the contour in the color gamut of the information-output apparatus can be smoothly continued. The image after conversion thus becomes more natural.
In the color gamut compression method of the present invention, a source color of information-input apparatus which is not in the color gamut of an information-output apparatus is converted to a target color inside a color gamut of an information-output apparatus and which is on a half line which passes a point corresponding to a color of an information-input apparatus. A convergence point is defined from one of a point at which saturation takes the value of 0, a point of an average value of the color gamut of a fixed tone in the information-input apparatus, a point of the central value of the color gamut of a fixed tone of the information-input apparatus, and a point of the weighted value of the color gamut of a fixed tone of the information-input apparatus. In this way, a simple calculation may be performed without impairing the continuity of the color of the information-input apparatus.
Thus it is possible to execute balanced color gamut compression processing with respect to tone and saturation and to convert an information-input apparatus to a color inside a color gamut of an information-output apparatus while maintaining continuity.
In the color gamut compression method of the present invention, a color of an information-input apparatus which is not in the color gamut of an information-output apparatus is converted to a color corresponding to the intersection point of a half line which corresponds to a color or an information-input apparatus and has an endpoint at a point of convergence and a contour of a color gamut of an information-output apparatus in any tone. In this way, it is possible to perform balanced color gamut compression processing with respect to tone and brightness.
In the color gamut compression method of the present invention, a source color of an information-input apparatus which is not in the color gamut of an information-output apparatus is converted to a target color according to three criteria. The first criterion is that a target color must reside within a predetermined angular range of the angle (xcex8, xcfx86) of a source color in a polar coordinate system (r, xcex8, xcfx86) of a color space. The second criterion is that the target color should be one of a plurality of colors on a contour of a color gamut produced by the information-output apparatus in any tone. The third criterion is that the target color should be obtained by dividing by kH, kV and kC, respectively, the H axis component, the V axis component and the C axis component of a color which minimizes color difference xcex94E given by the equation (4) below, where kH, kV and kC indicate predetermined coefficients corresponding to the H axis, the V axis and the C axis of a HVC color space, and xcex94H, xcex94V and xcex94C indicate an amount of displacement due to conversion of a color component in the H axis, the V axis and the C axis.
xcex94E={square root over ( )}(kHxc3x97xcex94H2+kVxc3x97xcex94V2+kCxc3x97xcex94C2)xe2x80x83xe2x80x83(4)
In this way, it is possible to convert a color after conversion to a color having characteristics based on the coefficients kH, kV and kC.
In the color gamut compression method of the present invention, when a tone of a color of an information-input apparatus is any one of a number of tones of a fixed number of standard colors, the tone is taken to be the corresponding tone from the plurality of tones and its brightness is taken to be equal to the brightness of the color with the highest saturation in a color gamut of an information-output apparatus for that tone. Furthermore a point where saturation takes a value of 0 is taken as an endpoint. When the tone of the color in the information-input apparatus is an intermediate tone with respect to the tones of a fixed number of standard colors, the point which is interpolated from the convergence point which corresponds to a tone of a fixed number of standard colors based on tones of colors of an information-input apparatus is taken to be the point of convergence. In this way, for example, the quality of the brightness of a standard color, distributed for example in low brightness, of a color from the blue group is reflected on the color after conversion. Furthermore it is possible to execute color gamut compression processing with higher continuity in the direction of tone.
In the color gamut compression method of the present invention, a vector which corresponds to a color in the information-input apparatus is defined as P and a vector which corresponds to a convergence point is defined as Po. A vector which corresponds to the intersection point of the half line which passes through a point which corresponds to the vector P taking vector Po as an endpoint and a curved surface which is formed on a contour of a color gamut in an information-output apparatus in each tone is taken as Ps. The vector which corresponds to the intersection point of the half line and the curved surface which is formed on the contour of the color gamut of an information-input apparatus in each tone is taken to be Pt. When the above conditions hold, the color of an information-input apparatus which corresponds to the vector P is converted to a color which corresponds to the vector Pc by Formula (5) when k has a value which is a fixed constant greater than 0.
Pc=Po+(Pxe2x88x92Po)xc3x97|Psxe2x88x92Po|/|Ptxe2x88x92Po|xc3x97k
(when kxc3x97|Pxe2x88x92Po|xe2x89xa6|Psxe2x88x92Po|)
xe2x80x83Pc=Ps
(when kxc3x97|Pxe2x88x92Po| greater than |Psxe2x88x92Po|)xe2x80x83xe2x80x83(5)
In this way, conversion proximate to the contour with respect to colors in the proximity of the contour in the color gamut of the information-output apparatus can be smoothly continued. The image after conversion thus becomes more natural.